1. Field of the Invention
This invention relates to the conversion of manganese dioxide to alkali metal permanganate and is more particularly concerned with the electrolytic oxidation of manganese dioxide to an alkali metal permanganate and the regeneration of permanganate etchant baths.
2. Descriotion of the Prior Art
Commercial processes for the production of potassium permanganate have included the production of potassium permanganate by oxidative roasting of potassium hydroxide and manganese dioxide to form potassium manganate followed by oxidation of the latter using chemical oxidants such as chlorine or by electrolytic means.
Kawasaki et al U.S. Pat. No. 4,117,080 teach the oxidation of manganese dioxide to alkali metal manganate and/or permanganate by oxidation of manganese dioxide dispersed in strong alkali metal hydroxide solution using nitrous oxides as oxidizing agent in the presence of a trace of manganate or permanganate.
Okabe et al U.S. Pat. No. 3,986,941 describe the preparation of alkali metal permanganate in high purity by electrolytic oxidation of a slurry of manganese dioxide or alkali metal manganate in caustic alkali having a concentration in the range of 10 to 25 percent by weight at temperatures higher than about 60.degree. C. These conditions (alkali concentration and temperature) are stated to be critical to successful operation of the process.
A particular need to convert manganese dioxide to alkali metal permanganate exists in the case of the regeneration of etchant solutions used in the treatment of plastics such as those used as substrates in the fabrication of printed circuit boards. Printed circuit boards vary in design but generally have a layer of copper laminated or otherwise held on either side of a plastic board, e.g. an epoxy resin board. In some cases the boards have multiple layers of copper separated from each other by epoxy and like plastic layers. In order to facilitate electrical connections between the circuits which are to be produced in the ultimate circuit boards, holes are drilled through the boards in appropriate locations and are subsequently plated with copper and the like to provide the necessary electrical connections. However, the formation of the through holes presents a plating problem in that the epoxy or other resin of the substrate is smeared on exposed copper during the drilling operation usually due to partial melting of resin by heat generated by the drilling.
Hence, prior to plating of the throughholes, the exposed resin in and around the holes is generally treated with a plastic etchant to improve adhesion of metal plating to the plastic. The most commonly employed etchant solution is an alkaline bath containing alkali metal permanganate as oxidizing agent. As the bath is used in the etching operation over a period of time a precipitate of manganese dioxide gradually accumulates as the by-product of the decomposition of the permanganate and as a by-product of epoxy resin etching. Eventually the etchant power of the bath falls to an unsatisfactory level and the bath can no longer be used.
Various attempts have been made to regenerate the bath and/or to improve its performance. A summary of such attempts is given in Courduvelis et al U.S. Pat. No. 4,592,852 which describes a method of improving permanganate etchant baths and prolonging their operating life by incorporating into the bath an effective amount of a secondary oxidant capable of oxidizing manganate ions to permanganate ions.
It has not previously been suggested that a permanganate etchant bath containing precipitated manganese dioxide could be regenerated electrolytically in situ to convert the manganese dioxide to alkali metal manganate and permanganate. The electrolytic regeneration of other etchants, namely, chromic acid baths employed to etch plastic materials such as ABS, has been described by Innes et al., Plating and Surface Finishing, November 1978, pp 36-40. In the process there described the chromous (Cr.sup.+3) salts produced in the bath in the oxidative etchant process are reconverted to chromic oxide by electrolysis of the strongly acid solution using a metal tin-lead anode and a cathode comprising a metal (copper) electrode immersed in 4.6N sulfuric acid solution contained in a porous ceramic container.